Phase modulation detector



April 2, 1946 y lM. G. cRoslasY` 2,397,841

PHASE MODULATIONXDETECTOR Filed April 5, 194s 2 sheets-sheet 41 A TTORNEY April 2, 1946.

M. G. cRQsBY PHASE MODULATION DETECTOR Filed April 3, 1943 2Sheets-Sheet 2 Patented Apr. 2, 1946 PHASE `MODULATION DETECTOR MurrayG. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application April 3, 1943, Serial No. 481,691

14 claims. n (C1. 25o-27) My present invention relates to phasemodulation detector circuits, and morepartcularly to improved forms of aphase modulation detector of the piezo-electric crystal type.

In the past I have disclosed various circuits for utilizing the inherentproperties of a simple crystal lter to convert phase modulation ofcarrier energy into amplitude modulation for detection. For example, inCommunication by Phase Modulation, Proceedings of the I. R. E., forFebruary 1939 (pages 126 to 136), I have shown a crystal lter phasemodulation translating network, and have explained the operation thereofon the basis of over and under-neutralization of crystal holdercapacitance.

It is one of the main objects of my present invention to provideimproved and modified types of crystal filter phase modulation A(l='Mhereinafter for brevity) detectors, in each type, or form, of circuitthe basic functioning being considered as involving the application toeach of a pair of opposed rectiers the resultant of crystal-filteredcarrier-energy and Vmodulated carrier energy in normal tion atresonance.

Another important object of my present invention is to detect PM waveenergy by a process which involves passing the PM energy through apiezo-electric crystal lilter to secure substantially unmodulatedcarrier energy, applying the unll tered PM wave energy to a pair ofopposed rectiers, shifting the relative phase between the filtered andunfiltered energy to quadrature rela- V tion for the resonancecondition, and applying the filtered energy to the rectiers thereby' tocause each rectifier torectify its respective vector resultant energy-Another object of my invention is to providea PM wave energy detectionnetwork for PM wave energy or AM (amplitude modulation) wave energy, thenetwork being capable of providing automatic frequency control (AFC)voltage, substantially pure carrier energy for carrier exaltation, andmodulation signal voltage.

Yet another object of my invention is to provide a device forcontrolling the selectivity of a crystal lter circuit feeding opposedrectiers of PM carrier wave energy.

Other objects of my invention are to improve generally the constructionand operation of PM or AM detectors of the crystal filter type, and moreespecially to provide such detectors in a simple andeconomically-manufacturable form.

The novel features which I believe to be characteristic of my inventionare 'set forth with particularity in the appended claims; `the inventionitself, however, as to both its organization and method of operationwill best be understood by phase quadrature relareference to thefollowing description, taken in connection with the drawings, in whichIhave indicated diagrammatically several circuit 0rganizations wherebymy invention may be carried into effect. In the drawings:

Fig. 1 shows one embodiment of the invention, Fig. la graphically showsthe'phase shifting characteristic of the crystal, j

Figs. 2a and 2b show vector relations between the filtered and unlteredsignal energy for the unmodulated and modulated states respectively,Fig. 2c vectorially explains the manner of phase detectionA from anotherviewpoint,

Fig. 2d shows theY vector relations between the `filtered and unfilteredsignal energy for a condition of off-tune, or olf-resonance,

Fig. 3 shows a modiiication of the circuit of Fig. 1,

Figs. 4 and 4a show two further modiiications,

Figs. 5 and 5a illustrate two additional modifications.

Referring to the 'accompanying drawings, wherein like referencecharacters inthe different gures designate similar circuit elements,there is shown in Fig. 1 one form of a detector circuit which embodiesthe features of the present invention. The detector network is providedwith an input transformer l whose primarycircuit 2 is resonated to thecenter, or mean, frequency of the applied modulated carrier wave energy.Since the present invention is not in any way concerned with the sourceof the signal Y energy. the circuits prior to the primary circuit 2 arenot shown in the drawings. The signals applied to the input transformerI may be PM, or AM, signal waves. Those skilled in the art are fullyaware of the various networks which could be employed prior to circuit2. Particular reference is made to my aforementioned publication for amore specific disclosure of such prior networks, where the receiver isof the PM type.y

It will be suilicient for the purposes of this application to assumethat the detector circuit is employed in a PM receiver of thesuperheterodyne type, and'that the carrier frequency ofthe transmittedPM wave energy will be some Predetermined frequency in the highfrequency band. For example, the band below 25 megacycles (mc.) isparticularly desirable for the radiation of PM signals. rIn thesuperheterodyne form of` reception, a local oscillation circuit isemployed to relower end of winding 1. v ergy may b e taken off from thecathode end of resistorv I4. Furthermore, that same point o f the outputresistor may be tapped for VAFC voltage Ithe carrier, and not of theamplitude variations of the latter.

Considering, now, the detector circuit per se, itis first pointed outthat the circuit comprises a pair of opposed rectifiers 3 and 4.These-rec` tiiiers are shown as of the diode type, since such rectifiersare simple in construction. However, the present invention is in nokwayv limited to the particular types of tubes shown, nor,/i n deed,`to

the above-mentioned specific frequencies 4which have been specified.IThe anode lof rectifier-3 'is connected to one end-of the secondarywinding 1 lof input-transformeri 'The last-mem' tioned :connectionincludes -inseries a piezo-electric crystal P and a resistor 8.

The crystal P is locatedbetweena pair of metallic electrodes in theusual fashion, and the l crystal is tuned Vto the mean frequency of theapplied signal energy. That is to say, the crystal .P istuned to theresonantV frequency of thefinput circuit 2. The electrodes of thecrystal may, if

desired,be metal coats vonthe opposite faces. l The resistor 8 maybe--shunted by an'adjustable condenser 8',and the function of the latterwill be .explained ata later point. The anode of rectiiier v4 isConnected to the opposite end r of the l secondary winding 1 ythrough apath comprising the capacity!! arranged in series with resistor I0.Resistor Ill is shunted by variable condenser I0.

The right-hand'terminalfof condenser 9 is conneoted by lead vII to jtheright-hand crystal electrode. Between the midpoint of Winding :'I and)the lead II thereis connected v*a resonant circuit which comprisesthevcoil ;I2 shunted by the yadjustable condenser I3. Normally, theresonant circuit I2-A-'I3 is tuned ,to the mean frequency value of theapplied-PM signal energy. The youtputload resistors of the circuit aredesignated byY numerals I4 and I5, and these resistors are connected invseries between the cathodes of diodes .3 ranelli. The cathode end'ofresistor Iisestabutilized for carrier exaltation detection in themanner disclosed in my U. S. Patent No. 2,063,588, granted December 3,1936. In such case, the pure carrier energy is fed to a separate phasemodulatiomor AM, detector. It is not believed necessary to show thedemodulator in such case, since those skilled in the art will readilyunderstand that it can be a seco-nd PM detector whose input is takenfrom'the input circuit 2 of Fig. 1. If desired, the'ltering of theenergy appliedto crystal P may be such as to leave modulation from Zeroto about 200 cycles on the 'carrier'. l Y

In explaining the functioning of the circuit Vshown* inFig. 1, it isfirst pointed out that the condenser S'is adjusted so as to neutralizethe capacity between the metal electrodes of crystal P. ,Referring tovFig. 2a there is shown the vector relations existing between theretarded and unretardedy voltages insofar as they affecttheopposedrectiiiers 3 and 4. The PM .signal energy which passes throughthe crystalPis stripped of its modulation side bands,Y so that there isapplied to the anode of each of recters 3 and 4 virtually unmodulatedcarrier energy. The vector Ep represents this crystal-filtered' carrierenergy appliedto the rectifiers. The path. from the crystal to therectier 3 is through resistor 8, while theV i path from the'crystal tothe rectifier 4 is through lead I I and resistor I0. It will, therefore,be seen that the filtered carrier energy at the output electrode ofcrystal P is applied Ain like polarity, or in parallel, to the anodes ofthe respective diode .rectiers A3 and 4. The passage of lthe filteredcarrier energy through resistors B and I to the respective detectorinput electrodes is accomplished without phase shift since the cou--pling is totally resistive except for the effect pro- .duced bycondensers 8 and I0. The effect of glished Vat ground potential, andAeach of resistors 1 i I4 and `I5 -is shunted by a carrier bypasscondenser. The junction of resistors I4 and I5 is connected by lead `llito the midpoint of the sec- 1 ondary winding ,'I.

A second path connects the anode o f each rectifier to the respectiveend of winding 1.

Thus, condenser 5 connects the anode of diode y 3 to the upper end ofwinding 1,'whi1e the condenser ,6 connects the anode of diode 4 to theModulation signal enin order to control the frequency of the .localOSilatOl, aS 'is Well understood. VVlhere AFC r voltage is taken'offfrom the cathode end .O f re- Vsister |,4f a modulation voltage filter.schermati cally represented by numeral I'I, is inserted in the. AFCoutputV line. Substantially pure, or` filtered, carrier energy isV takenoff from lead I8.

The filtered carrier energy will have a frequency equal to the meanvalue of the applied PM signal energy. Such ltered carrier energy may becondensersv" and I0' is compensated for by detuning I2-I3 as will bedescribed later.

The unfiltered PM signal energy which passes to each of rectiiiers-3land 4 through condensers l5 and 6 respectively is shifted 90 :degreesinphase by virtue of the capacity feedoto the respective 4detector inputelectrodes. Furthermore, since these energies are taken from theopposite ends of the winding I they are applied to the opposed1'ecti1iers'3 and 4 in polarity opposition. It will .be noted that themidpoint of winding 'I is effectively at ground potential with respectto radio frequenciesbecause the lead I6 connects the midpoint to groundthrough resistor I5. The

vectors Es and E5 denote the voltages applied to Y rectiers 4 and 3respectively, and it will be seen that these vectors are in phasequadrature with the crystal-filtered carrier energy. This phasequadrature relation of the two voltages at each rectifier results fromthel fact that the unfiltered signal energy is applied to the rectifiers3 and 4 by condensers 5 and 6 respectively which are sufiiciently smallto effect a degree phase shift, and are, also, of substantially equalcapacities so as to produceequalphase shifts both of the un- `filteredcarrier Yand of signal components. The

condensers 5 and 6 are non-selective to phase or frequency variations oflthe unfiltered carrier, and, accordingly, permit all signal componentsto pass to the rectiiiers 3 and 4. The crystal P, however, effects nophase shift at the carrier, but substantially removes the phasemodulation of the signal, thereby restoring the carrier substantially tothe phase and wave form which it had before modulation at thetransmitter.. TheV crystal P is, of course, selective for frequenciesoff resonance. This follows Afrom' the sharp selectivity characteristicofthe crystal, as depicted ideally in Fig. 1a. f f

rhe vector representing the resultant signal energyv at each rectifieris,also indicated in Fig. 2a. Thus, the vector Eo' represents theresultant energy applied to rectifier 3. The vectorEo represents theresultant energy applied to rectifier 4. Fig. 2a depicts the situationwhen the mean frequency value of the applied PM signal energy isinstantaneously equal to the frequency of circuit 2 and the frequency ofthe crystal P. The rectified outputs of each rectifier will, therefore,be equal, and the effective voltage at the cathode end of resistor I4will, therefore, be zero. In other words, for the in tune state no .AFCbias'is developed. l

In explaining the operation of this circuit, there are two separateconditions Which'must be considered. One is the case of the demodulationofvaf phase modulated signal, and the second is 'the case of thedetectionof slow frequency variations to obtain AFC potentials. Thesetwo cases represent two diierent degrees of modulation that are actedupon bythe crystal filterin different-v manners. For the case of therelatively rapid modulation represented by the phase modulations of thesignal, the lter'acts' asa device 'which selects the carrier from theside bands, and provides the equivalent ofa synchronized local carrierfree of modulation. When an unmodulated carrier is received there aredelivered to each of diode rectifiers 3 andV 4 voltages, Yone a filteredcarrier from crystal P without phase change, and the other an unfilteredcarrier substantially 9G degrees different in phase from the filteredcarrier. When the 'received carrier is phasemodulated the filteredcarrier remains as before, but the unfiltered signal energy-is suppliedto the rectiers 3 and 4 in phases differing from the 90V degree, orquadrature,l relation to an extent determined by the degree 'of phasemodulation. If the degree of phase modulation is small, a relativelysmall direct current voltage is built' up across rectifier outputresistors I4 and I5 due to the signal voltage increasing on oneof therectifiers and decreasing on the other. The greater the degree of phasemodulatim` the greater the combined voltage'of the unfiltered signalenergy and the filtered carrier from crystal P on one of the dioderectifiers, and the less th'esuin of suchv voltages on the otherrectifier. The polarity of the direct current voltage drop across theload resistors I4 and I 5 of the opposed rectiers depends on thedirection of the phase change of the receivedsignal energy.

For the caseof the relativelyslow variations in frequency of theincoming signal, the crystal filter acts as a retard circuit having anoutput phase which varies with the' frequency of the input. For thiscase, thecircuit acts like a very narrow-band frequency modulationdiscriminator. The solid curve in Fig. la-showsthe frequency vs. phaseshift characteristic of crystall P. At Fc, the center frequency, thecrystal provides zero phase shift. v

The vector diagrams of Figs.v 2a, 2b andl Zcshow the conditions for thecase of phase modulation detection. The ltered carrier, `which isrepresented by vector Ep, remains fixed in'l phase. .The unfilteredmodulated signals, represented by vec. tors E5 and Ee, vary in phase tovproduce differentially modulated resultanteI En and En', which are fedto the opposed detectors.-rv y which the modulated signal variesr inphase with The manner in,

respect to the filtered carrier is shown in-Fig. 2c.

A condition of'smodulation in- 'one direction is shown in Fig. 2b. Theunmodulated condition is shown in Fig. 2a.

The -vector diagrams of Figs.l 2a and 2d show the conditions for thecaseof AFC detection. Fig.

v2a shows the in-tune condition which is effected when the appliedsignal carrier frequency is in the middle of the crystal filtercharacteristie. Y The diagram of Fig. 2d shows the relations for anolf-tune condition. It will be noted that the carrier (or crystaloutput) phase shiftsvfor the 'off-tune condition. This phase shift is Ybrought about by the phase characteristic of the elements.

crystal, which is similar to that of an ordinary resonance circuit, asshown in Fig. 1a. The magnitudey and sense of phase shift of thefiltered carrier energy are respectively dependent on the amount anddirection of frequency departure' of the modulated carrier energy atcircuit 2 relative to the predetermined frequency Fc. The latter is, ofcourse, the resonant frequency of crystal P. The signal energy passingthrough condensers 5 and' will not shift in phase in response to carrierfrequency departures from Fc. This follows from the fact that condensers5 and 6 are non-selective Hence-and as shown in Fig. 2d, the resultantvector voltages EnV and En will vary in relative magnitude depending onthe extent and sense of theaforesaid frequency departure. rIheserelative variations in En and En' are translated into correspondingdirect current voltage variations across load' resistors I4 and I5, andthe differential of these direct current voltages is used asVAFC biasafter filtering at I1. It is seen that the carrier phase for theoff-tune condition is no longer in its proper quadrature relationshipwith the unfiltered signal so. that itmight be thoughtthat the detectionof phase modulation would be impaired. However, this off-tune conditionis never allowed to exist to any appreciable degree. since the AFCcircuit functions to correct the tuning and maintain it in the in-tunecondition represented by Fig. 2a.

. Winding 'l has its midpoint grounded, as pointed out above, andparallel resonant circuit I2, I3 connects the grounded midpoint of coill to the lead II which connects the output electrode 'of the crystal Ptothe rectiflers. The circuit I2, I3 is. accordingly, in effectconnected between `the outputl side of crystalP and ground. Circuit I2I3 may be tuned to crystal frequency, or may be detunedrelativelythereto, and acts as a .coupling circuit of finite impedance between theoutput of crystal P and thefrectiiiers 3 and 4. The circuit I2,I3'incre'a`ses the Q of the crystal beyond what it would be if theresistance of circuit vI 3 were infinite. By detuning resonant circuitl2--I 3 the phase ofthe filtered carrier energy can be shifted to apredetermined extent. This phase shift can be compensated by each shuntcapacity 'and I0'. Each of resistors 8 andl I 0 is; there- ,5, andresistor Il) and condenser Iii' may be interchanged withcondenser 6.This would in no way aiect the relative normal phase quadrature i--relationhetvreonitheireterdodand mretarsodrPM .;sier,1.al;enereya,sdepicted FierZa.

a mod'caticmp-.tho

glue".

Aner as shown in Fig.'

arrangement in Fig. 1, wherein he re "Store 8 and l0, are,.rop1aoedgbrresrzeogt've o onyensers 8" f1 The unlteredtRM. signal. lenereyin thatoase -is derived .from 'the primary-,Circuit ':2-

neotiondeading. tothe upperzendrof thezseoondarv tered modulatedcarrier., energy, is iderivedby informersecondary circuit..`SinceitheS-anodezconvnections are made to the. o ppsite .endsuof1theSince at ,resonance theprimary volta-ge hearsga Y,

phase quadraturerelation withrespect` torthe Secondary voltage. rit Willbe seen :that the ,reoti-l fiers .3 and Lvvill have nropenrhase,relations vlootwoon the filtered-and nnltorodisienolenerey.

The numeral, lI s designa-tesori; ampler tube, :or

an amplitude limiter-tube, whose' platev-'oirouitfinoludes vthe resonantprima e. vplate foffamplier vl 8 -mittod through condonsersj. andrespectively will'be lin polarity opposition-When;applied tothe-1anodesiof reotiers-S and il. The selectivitycolo- 1. The anodei eachrectier is connected to the jnncgtiongof load resistors .I4 andv 15through respective;returrmesstots 414V :and I5'. f

secondary' winding.v The condenserq .connected vbetween the Vmidpoint ofthe secondary -`V v vinding vand the lower end ofy the primary Winding-functions to neutralize the crystal interelectrode iCarpacity. The purecarrier energy is takenirom the right-'handelectrode of the crystal.'Other` wise, Athe circuit functions -in-lthesameqmanner as described inconnection with `1ig.;r1. .It is suf- -cient topoint out in connectionwith this modication that the filtered PM 'signa-1 energyiis appliedtothe anodes of rectifier-S A'3i-and# in like polarity by virtue ofthevconnection vofthe crystal `to the midpoint ofY the secondary winding.-On the' other hand. ythe unltered signal energy `is first shifted 90degrees in phase :bythe rmagnetic coupling M, and-'the phase shiftedenergies are applied in polarity vopposition to-'the anodes-of theopposed rectiers'; -It 'mayhappen that the circuit shown in Fig. .4 willgive rise to lsecond harmonics in the pure carrier energy taken off fromthe crystal iP. Y This -is fbroughtiahout *by full-wave rectifiedvoltage', rectifledsfrom the secondary of' transformer 4I :and`falpearing acrOSS 12,13.

-In th at case,ltherectiers 3 :and 7,4 are reversed `in connections to.eliminate the .Afullfwave connection, and cause .therectier Ato conductsimultaneous'y instead of alternately and Vthereby suporess theproduction ofsooond harmonics. This `if; ldone by connecting the loadresistors ,1,4 and in the manner shown in.1ig. .4a. VvThe anode ofrectifier is connected to the junctiQn of resistors i4 and 15, while theveathode of diode 3 is connected to the upper .end .of resistor .1.4. Inother words, the onlychange that needbe made in the circuit of Fig. .4Ais, that indicated with respect to resistor I4 .and its associateddiode rlhe diode-resistor condenser is nolonger across resistor I4, but.takes .the .form .of .condenser '.3'

ductive couplingr-fromthegprimaryrcircuit 2 ,1 and is applied; to therrectifers3sandzl lfrom the transsecondary 1, Ywhose midpOint Liseiectivelyfconnected 4toground 'for radio frequencies; itzwill ;be

seen that the unfiltered signal enereyig applied t0 the Opposed rectiersY.in ;.pol arity ioppositiou. The ltered carrier ener-gm,` however,.isapplied Y to the diodesgin like phase. 'lyhehlterdcarlier energyis.'y taken Volif froin: the :leftfhafnd #terminal erges are bothderived from the secondarycrcuit -oi theinput-transformer..fHoWever,the-;required 90 degrees phase :shtis :secured fini. thiszmodcatiollrio-y.v deliberately tdetuning the :resonant;.cir

cuit i3-H2. .-lhedetuningissoeadiusted-ras:to

secure the,. 90 degrees .phase retardation rIncidentally, the f crystalselectivityfisjncreasedupon detuningof-fthe resonant-circuit. `j-. Inother words, with a circuitfof theiormeshown in Fig.; 5;the;net

. work Ir2-t3 not only 4functions?as@arphasexshifter element, Jb ut'also facts; as f a gcontrpl: element over lfthe selectivity ofthecrystaljlter.

In Fig. 5a thereyis shown t;he imodication fwhich is i required in Jig.V5, such 1asztolsuppress lthe* presence; of :second harmonicszin-ztheltered carrier energy. vnithispase theffanode of diode 4 isconnected tothe grQundedzrendsof load -resistor l5, while the fcathodefwill"beyconnectedto thelower endofsecondaryrwinding 1. Thefanode0f diode 3 Willl be connected tothe upper :end 'of resistor I4, and itscathode: will fbe connected to they junction of resistors Maand slii.Itvvilliibe noted that -in vboth 'Figsfsia'and Sarthe- -rectiers arearranged to conduct .simultaneouslyrather than Valternately when -sfedhy fthe. .push-pull secondary of transformer `I.

' While I -vhaveV indicatedan'd .described several ysystemsl forcarrying kmyiinvention .into effect, '-it will be apparent to oneskilledzin'the-artthatrmy invention is by nomeansI limited to`theiparticular organizations `shown .:anddscribed, :but that -many-modifcations may be :.madev without sdeparting from -Athe .fscoperof:my invention, -fas Eset forth inthe appended claims.

What-Iolaim is: i Y

1. A method of .detecting'modulatedcarrier energy which comprisesremoving substantially. all the modulation lfroma I:portion of theenergy, dividing the -unmodulated-fenergy into two portions of likepolarity relation, 1dividingthe original modulated energy Avin-to two4:additional portions of opposite polarity relation, displacing therelative phases '.loetween :said two :unrno'dulated portions andxsaidrtwo ninodulated ,portions to the vextent of substantially f90degreesfat resonance,

separately combining in pairs an iurrmodulated portion and modulatedportionin-phase ii-isplaced relation, separatelyr rectifyinglthefresfultant of each of the combined'pairsfand' combining therectication productsof the separate rectication lStoiosin opposition. i

' 2. *Ina detector of Ipdlllatfd sig-naHingenergy,

'a vsignal input ttansormer havinga vprimary resonantcircuittunedtothegnean treguency 0iA applied modulated,signalenergy, .asecon-darycircuit including -a piezo-electric .crystal element ytuned to thas-aidmean freouenoy, anrstrtectiiierpmeans ,connecting the rectiiierelectrodes .i-n .circuit :with

said crystal element thereby to'have the crystal output energyapplied-thereto, a second rectifier in circuit -with the crystal elementhaving said output energy applied thereto in like polarity, a commonoutput circuit connecting said rectiers in polarity opposition, separateconnectiongfr'om respectively separated points of the input transformerto the respective'rectiilers for applying thereto uniilteredmodulatedsignal energy, said separated points being of 'opposite polarity; andmeans for producing a normal phase quadrature displacement betweenthe'crystal output kenergy and the unfiltered energy.

3. In a detector of phase modulated signalling energy, a signal inputtransformer having a primary resonant circuit tuned to the meanfrequency of applied signal energy, a secondary circuit including apiezo-electric crystal element tuned to the said mean frequency, a rstrectifier, means connecting the rectifier electrodes in circuit withsaid crystal element thereby to have the crystal output energy appliedthereto, a second rectifier in circuit with the crystal element havingsaid output energy applied thereto in like polarity, a common outputcircuit connecting said rectiers in polarity opposition, separateconnections from respectively separated points of the input transformerto the respective rectifiers for applying thereto unfiltered phasemodulated energy, said separated points being of opposite polarity, anda resonant circuit, normally tuned to said mean frequency, in circuitwith the crystal element, said resonant circuitincluding means foradjusting its frequency whereby the phase relation between the crystaloutput energy and the unfiltered energy may be'varied.

4. In a detector of phase modulated signalling energy, a signal inputtransformer'having a, primary resonant circuit tuned to the meanfrequency of applied signal energy,'a secondary circuit including apiezo-electric crystal element tuned to the said mean frequency, a firstrectifier, means connecting the rectifier electrodes in circuit withsaid crystal element thereby to lhave the crystal output energy appliedthereto, a second rectifier in circuit with the crystal element havingsaid output energy applied thereto in like polarity, a common outputcircuit connecting Said rectiers in polarity opposition, Separateconnections from respectively separated points of the input transformerto the respective recti'ers for applying thereto unfiltered phasemodulated energy, said separated points being of opposite polarity, saidseparate connectionsI each including a condenser, and said separatedpoints being on the primary resonant circuit to provide a normal phasequadrature displacement between said crystal output energy and theunfiltered energy.

5. In a detector of amplitude modulated signalling energy, a primaryresonant circuit tuned to the mean frequency of applied signal energy. asecondary circuit including a piezo-electric crystal element tuned tothe said mean frequency, a first rectifier, means connecting therectifier electrodes in circuit with said crystal element thereby tohave filtered output energy applied thereto, a second rectifier incircuit with the crystal element having said filtered energy appliedthereto in like polarity, av common output circuit connecting saidrectiers in polarity opposition, separate connections from respectivelyseparated points of the input transformer to the respective rectifiersfor applying thereto unfiltered modulated energy, and means forproducing a. normal phase quadrature displacement between the n1- teredenergy and the unfiltered energyat Aresonance. t f

6. In a detector of phase modulated or'amplitude modulated signallingenergy, a signal input transformer having a. primary resonant circuittuned to the mean frequency of 'applied signal energy,A a secondarycircuit including a piezoelectric crystal element tuned to the said meanfrequency, a first diode rectifier, means connecting the diodeelectrodes in circuit with said crystal.v element thereby to have thecrystal routput energy applied thereto, a second diode rectifier incircuit with the crystalelement having said output energy appliedthereto, a 'common output circuit connecting said rectiiers, separateconnections from respectively separated points ofthe input transformerto the respective rectiiiers for applying thereto unfiltered phasemodulated energy, said separate connections each including a condenser,and said separated pointsl being on the primary resonant. circuit toprovide a normal phase quadrature displacement between said crystaloutput energy and the unfiltered energy.V

7. A methodvofdetecting phase modulated or amplitude modulated carrierenergywhich comprises removing substantially all tne modulation from aportion of the energy by crystalfiltering. dividing the filtered energyinto two portions, separately dividing the original modulatedenergy intotwo additional portions, providing a. phase shit between the filteredyand unltered original energies such that the relative phases betweensaid two filtered portions and said two'iinfilt'ered portions issubstantially degrees, combining the phase displaced portions inpairsseparately rectifying the combined pairs, andemploying thedifferential resultant of the separate rectifications. j

8. In a detector of modulated signalling energy, a signal inputtransformer having a primary resonant circuit tuned to the meanfrequency of applied signal energy, a secondary circuit including apiezo-electric crystal element tuned to the said mean frequency, meansfor neutralizing the crystal inter-electrode capacitance, a firstrectier, means connecting the rectifier electrodes in cir. cuit withsaid crystal element thereby to have the filtered crystal output energyapplied thereto, a second rectier in circuit with the crystal elementhaving said filtered output energy applied thereto in like polarity, acommon resistive output circuit connecting said rectliiers, saidrectifiers being arranged to suppress second harmonies of the filteredenergy, separate connections from respectively separated points of theinput transformer to the respective rectiers for applying theretounfiltered modulated energy, said separated points being of oppositepolarity, and said separated points being located on the transformer toprovide a, normal phase quadrature displacement between said crystaloutput energy and the unfiltered energy.

9. In combination in a signal transmission network having a pair ofinput terminals and a pair of output terminals, a first path couplingone input terminal to its corresponding output terminal, said pathincluding a piezo-electricv crystal element of a predeterminedfrequency,l a second path in shunt with the first path and. having anon-selective characteristic with'respect to signals, and a resonantcircuit, tuned substantially to said frequency, in circuit with said twopaths, said resonant circuit including means for adjustlng thefrequencythereof thereby to control the selectivity of said crystaldomi-Pnt path,

